The present invention generally relates to liquid crystal display devices and more particularly to a liquid crystal display device having plural sub-picture element electrodes in a single picture element region and a method of suppressing afterimages therein. Particularly, the present invention relates to a liquid crystal display device in which at least one sub-picture element electrode is coupled to a control electrode to which a display voltage is applied via capacitance and the method of suppressing afterimages, or burning of images, in such a liquid crystal display device.
A liquid crystal display device has a compact size in terms of thickness as compared with CRT (cathode ray tube) and has an advantageous feature of low drive voltage and small power consumption. Thus, liquid crystal display devices are used extensively in various electronic apparatuses including television sets, laptop personal computers, desktop personal computers, PDAs (personal digital assistants), cellular phones, and the like.
Particularly, a liquid crystal display device of active matrix type provided with a thin-film transistor (TFT) for each pixel (sub-picture element) for switching element, can provide the display characteristics comparable to those of CRTs in view of its high driving performance, and liquid crystal display devices are now used extensively in the applications where CRT has been used conventionally, such as desktop personal computers or television sets.
Generally, a liquid crystal display device is constructed by two substrates and a liquid crystal confined therebetween. There, one substrate carries picture element electrodes and TFTs thereon in correspondence to the picture elements. On the other substrate, there is formed a common electrode commonly to the picture elements and color filters are formed further thereon in correspondence to the picture element electrodes provided on the first substrate. There are three filter types: red (R); green (G); and blue (B) in such color filters and each picture element carries a color filter of any one color.
Thereby, three picture elements of red (R), green (G) and blue (B) disposed adjacent with each other constitute together a pixel. Hereinafter, the substrate carrying the picture element electrodes and the TFTs will be designated as TFT substrate, and the substrate disposed over the TFT substrate will be designated as opposing substrate. Further, the structure in which a liquid crystal is confined between a TFT substrate and an opposing substrate is designated as a liquid crystal panel.
Conventionally, a so-called TN (twisted nematic) mode liquid crystal display device has been used extensively, wherein a TN mode liquid crystal display device confines a horizontally aligned liquid crystal (a liquid crystal having a positive dielectric anisotropy) between the two substrates with twisted alignment of the liquid crystal molecules.
However, such a TN mode liquid crystal display device has a drawback of poor viewing angle characteristics in that there is caused a remarkable change of contrast and tone of color when the images on the liquid crystal display device is viewed from an oblique direction.
Under these circumstances, there is developed a so-called MVA (multi-domain vertical alignment) liquid crystal display device, wherein a MVA liquid crystal display device is characterized by excellent viewing angle characteristics and is already used in practice.
In such a conventional MVA liquid crystal display device, however, there still occurs a phenomenon that the representation appears whitish when viewed from an oblique direction.
FIG. 1 is a diagram showing a T-V (transmittance-voltage) characteristic of an MVA liquid crystal display device according to a related art for the case of viewing an image represented thereon straight from the front direction and for the case of viewing the image from a direction offset in the upward direction by 60 degrees. In FIG. 1, the horizontal axis represents the applied voltage in terms of volts while the vertical axis represents the transmittance.
As can be seen in FIG. 1, there can occur a situation in which the transmittance takes a larger value when the image is viewed from the oblique direction as compared with the case of viewing the same image from the front direction when a voltage slightly larger than the threshold voltage is applied to the picture element electrode.
FIG. 1 also indicates that the transmittance for the case of viewing the image obliquely becomes smaller than the transmittance value of the case of viewing the image straight, when the applied voltage has exceeded a certain level.
Thus, the difference of brightness between the red picture element, green picture element and blue picture element is decreased when viewed from the oblique direction, and this is the reason why the foregoing problem of whitish representation occurs.
This phenomenon is called discoloration, wherein it should be noted that discoloration takes place not only in MVA liquid crystal display devices but also in TN mode liquid crystal display devices.
U.S. Pat. No. 4,840,460 proposes a technology of dividing a single picture element into plural sub-picture elements and couple sub-picture elements by way of capacitance coupling.
With the liquid crystal display device of this type, the voltage is divided by the ratio of capacitance of the respective sub-picture elements, and it becomes possible to apply different voltages to the respective sub-picture elements. With such a construction, therefore, there apparently exist plural regions of different threshold values of T-V characteristics within a single picture element.
As a result of existence of plural regions having respective, different threshold values of the T-V characteristics in a single picture element, the problem that the transmittance value increases when viewed from an oblique direction over the transmittance value attained for the case of viewing the liquid crystal display device straight from the front direction, is effectively suppressed.
It should be noted that this technology of improving the display characteristics by dividing a single picture element into plural sub-picture elements coupled with each other by way of capacitance coupling is called HT (halftone) gray scale method attained by capacitance coupling. Meanwhile, it should be noted that the liquid crystal display device disclosed by the U.S. Pat. No. 4,840,460 is a TN mode liquid crystal display device.
Japanese Patent 3,076,938 or corresponding Japanese Laid-Open Patent Application 5-66412 discloses a TN mode liquid crystal display device in which a picture element electrode is divided into plural sub-picture element electrodes, and a control electrode is disposed underneath the sub-picture element electrodes via an insulation film.
With this liquid crystal display device, a display voltage is applied to the control electrode via a corresponding TFT. There, because the sub-picture element electrodes have respective, different sizes, mutually different voltages are applied to the sub-picture element electrodes, and as a result, discoloration is suppressed by the effect pertinent to the HT method.